Fiber-reinforced composite materials composed of a reinforcement fiber and a matrix resin have light weight and exhibit excellent dynamic properties. Therefore, they are widely used in sporting-goods applications, aerospace applications and general industrial applications. The reinforcement fibers that are used in these fiber-reinforced composite materials assume a variety of forms in accordance with the use and reinforce molded articles. As such reinforcement fibers, for example, metal fibers such as aluminum fibers and stainless steel fibers, organic fibers such as aramid fibers and PBO fibers, inorganic fibers such as silicon carbide fibers, and carbon fibers are employed. From the standpoint of the balance in the specific strength, specific rigidity and light-weightness, carbon fibers are preferred and thereamong, polyacrylonitrile-based carbon fibers are suitably employed.
Further, as a matrix resin used in these fiber-reinforced composite materials, for example, thermosetting resins and thermoplastic resins are employed, and these resins are each produced by a different method.
As a method of producing a fiber-reinforced composite material in which a thermosetting resin is used as a matrix resin, for example, a method in which a plurality of prepregs, each of which is a sheet-form intermediate material in which a reinforcement fiber is impregnated with an uncured thermosetting resin, are laminated and then heat-cured, a resin transfer molding method in which a liquid thermosetting resin is poured into a reinforcement fiber provided in a mold and then heat-cured, a filament winding method in which a reinforcement fiber, which is immersed in and impregnated with a liquid thermosetting resin, is wound around a mandrel or the like and then heat-cured, or a pultrusion method in which a reinforcement fiber is immersed in and impregnated with a liquid thermosetting resin and then passed through a heated mold, thereby heat-curing the thermosetting resin, is employed.
In general, as compared to thermoplastic resins, thermosetting resins have a higher elastic modulus. However, they are inferior in terms of toughness and durability. Among thermosetting resins, epoxy resins have been preferably used from the standpoint of adhesion with a reinforcement fiber and, as a method of improving the toughness and durability of an epoxy resin, there have been tried methods of blending a thermoplastic resin therein. However, in those methods, since the viscosity of the resulting resin is largely increased, there are problems of deterioration in processability and reduction in quality caused by void generation or the like.
For example, there is proposed a method in which a copolymer of styrene-butadiene-methyl methacrylate or a block copolymer of butadiene-methyl methacrylate or the like is added as a thermoplastic resin to allow a fine phase-separated structure to be stably formed during the process of curing an epoxy resin, thereby largely improving the toughness of the resulting epoxy resin (WO 2006/077153).
Further, for example, there are known a method of preparing a prepreg in which a polyarylene sulfide is made into the form of a slurry in a dispersion medium to facilitate impregnation thereof into a glass fiber mat (JP H5-39371) and a method of producing a laminate without using a prepreg by preparing a sheet of a polyarylene sulfide having a relatively low molecular weight and laminating the sheet with a fiber base material (JP H9-25346).
Meanwhile, as a method of producing a fiber-reinforced composite material in which a thermoplastic resin is used as a matrix resin, there is known a method of producing an arbitrary molded article by using a molding material obtained by impregnating a reinforcement fiber with a thermoplastic resin, such as a prepreg, a yarn, a glass mat (GMT), a compound pellet or a long fiber pellet (for example, JP 2000-355629, JP 2003-80519 and JP 2010-121108). Since such molding material is easily molded because of the properties of thermoplastic resin and does not undergo curing during storage as in the case of thermosetting resins, such molding material does not impose a burden of storage and characteristically yields a molding article having high toughness and excellent recyclability.
In the method disclosed in WO '153, since the viscosity is largely increased by addition of a thermoplastic resin, processability of the resultant tends to be markedly impaired. Therefore, to minimize the effect on the processability, the amount of the thermoplastic resin to be added must be reduced so that the method is not likely to be able to impart an epoxy resin with sufficient toughness.
In the method disclosed in JP '371, not only equipment and time are required to dry a dispersion medium, but also it is difficult to remove the dispersion medium completely. Therefore, the method has a problem in that sufficient mechanical properties are not attained due to voids that are generated by evaporation of the dispersion medium at the time of lamination and molding. In addition, the method disclosed in JP '346 has problems in that the molding is required to be carried out at a high temperature and high pressure and that satisfactory mechanical properties are not attained due to a defect such as insufficient impregnation.
Furthermore, when a thermoplastic resin is used as disclosed in JP '629, JP '519 and JP '108, since the shape thereof cannot be retained at its melting temperature, the resin must be cooled in a mold so that there is a problem of a decrease in the cycle efficiency. Moreover, there is also known a method in which a melted resin is molded by pressing simultaneously with cooling thereof. However, since such a method requires a melting/heating apparatus to be introduced for melting and heating, there is a problem of an increased equipment investment.
In view of the above-described problems of prior art, it could be helpful to obtain a resin composition having good moldability and impregnation properties. It could also be helpful to provide a composite cured product comprising the resin composition which contains a reduced amount of voids and can be demolded even at its curing temperature.